1. Field of the Invention
This invention relates generally to methods of sampling particles, especially carbon black, for measuring fineness, or particle surface area, and methods of controlling carbon black reactors.
2. Background
The current method of determining carbon black fineness (surface area) during the manufacturing process is to collect a sample, take it to the lab, and then determine the fineness via I2 or N2 absorption methods. This gives a delay time of at least about an hour. Therefore, several hours of “off-spec” carbon black could be produced while “lining out” the reactor conditions (adjusting reactor conditions in response to the fineness measurements which come from the lab to bring the carbon black into specification), since several iterations are usually required to achieve the product specification targets.
There has, therefore, been a long felt need in the carbon black industry for in-situ sampling and measurement of carbon black fineness during the manufacturing process so that adjustments can be made to the process more quickly. It is desired to provide real-time, on-line sampling and fineness (particle surface area) measurements of carbon black while the carbon black is being manufactured.
Laser-induced incandescence (LII) has been used as a soot diagnostic technique since about the 1980s. The basic principle of LII is to rapidly heat up particles with ultra-short laser pulses (laser pulse is typically <20 ns duration) of high energy. Particle temperature is increased to a point to produce significant incandescence of the particle, or even up to vaporization temperature (for carbon blacks, about 4000 K). Particles lose this added energy via 3 paths: vaporization, heat conduction to the surrounding medium, and thermal radiation. The enhanced thermal radiation is then detected (emission signal). The incandescence from the particles is measured using collection optics and photo detectors. Using appropriate calibration and analysis of the incandescence signal, information such as the soot volume fraction (svf) or the primary soot particle size may be obtained. The method is essentially non-intrusive and is capable of making in-situ measurements.
LII measurement is an emerging technology that has promise to be a reliable means for spatially and temporally measuring the concentration of carbonaceous particles and their spherule size. LII has been developed primarily for monitoring particulate emissions produced by combustion of hydrocarbon fuels. In the past 10 years or so, academic researchers have utilized LII to resolve spatial concentrations of soot in laboratory flames and diesel engines (See, e.g., Dec, J. E., zur Loye, A. O., and Siebers, D. L., “Soot distribution in a D.I. Diesel Engine Using 2-D Laser Induced Incandescence Imaging,” SAE Transactions, 100, pp. 277–288, 1991).
LII is suitable for soot particulate measurements since the LII signal is proportional to particulate volume faction over a wide dynamic range. LII provides a relative measure of soot concentrations and requires a calibration for quantification of soot particulate concentrations. LII has been used to measure soot particle volume fraction in steady-state and time-varying diffusion flames, premixed flames within engines and in diesel engine exhaust streams, and gas turbine exhausts. These LII applications are with relatively dilute (low concentration) streams of soot.
Recently, a technique for performing absolute light intensity measurement in LII has been presented, thus avoiding the need for a calibration in a source of soot particulates with a known concentration (U.S. Pat. No. 6,154, 277), and, thus, extending the capabilities of LII for making practical quantitative measurements of soot. Using this in-situ absolute intensity self-calibration technique, LII has been applied to measure soot particle volume fraction in laminar diffusion flames, carbon black, and in diesel engine exhaust streams. See, e.g., Snelling, D. R., Smallwood, G. J., Gülder, Ö. L., Liu, F., and Bachalo, W. D., “A Calibration-Independent Technique of Measuring Soot by Laser-Induced Incandescence Using Absolute Light Intensity,” The Second Joint Meeting of the U.S. Sections of the Combustion Institute, Oakland, Calif., Mar. 25–28, 2001.
It has also been theorized that LII could be used to measure primary particle size. Some work toward using LII for size (sample particle diameter) measurements of soot and carbon black were published by various academic groups. See, e.g., U.S. Pat. No. 6,181,419; WO 97/30335; and Starke, R. and Roth, P., “Soot Particle Sizing by LII During Shock Tube Pyrolysis of C6H6,” Combustion and Flame, 127:2278–2285 (2002) (the disclosures of which are hereby incorporated by reference for their general teaching on LII methods for determining particle size and LII apparatus/instrumentation used in determining particle size).
For determination of soot concentration, the analysis of the incandescence signal at one point in time (just after the laser pulse) is usually sufficient. However, since heat conduction is mainly governed by particles' specific surface areas, the cooling rate is a characteristic measure for primary particle size, since larger particles will cool more slowly than smaller particles (note: cooling rate time constants are on the order of 1000 ns). The determination of particle fineness requires that the incandescence signal be measured as a function of time while the particles cool. Basically, the dependence between signal decay time and primary particle size is proportional, i.e., smaller particles show lower decay times, but it is generally not linear. Time-resolved LII (TIRE-LII) yields primary particle size by comparing measured temporal signal decay to calculated decays. In order to increase the precision of the technique, since a single data point is collected very quickly, it is common to average the incandescence data from many laser pulses. A typical set up may use a laser with a 20 Hz repetition rate and average the data from 40 pulses, giving a single data point every 2 seconds.
A photomultiplier can be used to measure the temporal signal behavior. The signal is recorded with a fast oscilloscope connected to a computer. Data is read out and a fit provides the characteristic signal decay time. This time is unambiguously connected with primary particle size under certain environmental conditions. If capturing the exact value of primary particle size is desired, known ambient conditions, particularly temperature, are required. The detection of change requires fairly constant conditions or accordingly, information about the temperature change.
Aside from any difficulties in choosing a method that provides real-time on-line measurements, many problems are present in providing particle samples to a chosen method. In using the LII technique for in-situ measurement, appropriate in-situ techniques for pulling and preparing the sample of carbon black must be provided in order to accurately and consistently perform the in-situ measurements.
Problems with sampling, adjusting samples, and measuring samples include plugging of lines used to sample, consistent sample dilution, moisture condensation in sampling lines, and fouling of optical windows.
Once in-situ, real-time sampling and measurement of carbon black can be performed, the tools for real-time process control of carbon black processes are available. By solving the problems of the prior art, the present invention is able to more quickly and reliably control carbon black production processes.